Electrodeposition of a corrosion resistant coating



United States Patent 3,247,082 ELECTRODEPOSITION OF A CORROSION RESISTANT COATING Arthur H. Du Rose, Euclid, Ohio, assignor to The Harshaw Chemical Company, Cleveland, Ohio, a corporation of ()hio No Drawing. Filed Aug. 7, 1962, Ser. No. 215,270 3 Claims. (Cl. 204-41) This application is a continuation-in-part of my copending application, Serial No. 11,450, filed February 29, 1960, and now abandoned.

This invention relates to corrosion resistant composite metallic articles and to high throwing power electrodeposition methods of producing said articles.

Considerable elfort has been devoted to the production of corrosion resistant plated metallic articles and particularly to corrosion resistant nickel plated articles. Nickel is especially suitable for corrosion prevention in that it forms coatings which are more or less of a permanent white appearance and possess a high degree of resistance to corrosion by natural waters, sea Water and alkaline and salt solutions. Nickel coatings, however, are also subject to corrosion due to certain phenomena existing in the structure of the coating and in the relation existing between the various components of the: coated article.

Corrosion of nickel andnickel-chromium plated articles is primarily due to two factors. One of these factors is the so-called initial porosity of the coating. The initial porosity may be defined as small pores which are present in the coating layer before any corrosion has taken place. This type of porosity can be minimized by (a) proper base preparation, (b) absence of insoluble matter in the plating solution, and (c) use of thicker deposits. For nickel deposits greater than 1 mil in thickness the initial porosity is usually very low.

The second factorcausing corrosion is galvanic in nature and is usually associated with discontinuities at the surface of the coating. These discontinuities or areas of galvanic action can be due to cracks or pores in the chromium deposit or to particles of air borne dirt catching on the surface. Due to moisture and the presence of various electrolytes therein, a galvanic cell is set up in the area of the discontinuity which gradually causes the nickel to be dissolved and a hole or pore to develop. The type of nickel and chromium coating and the atmosphere to which the coating is subjected will affect the rate of galvanic penetration. It is important to note that thickness is again an important factor. The heavier deposits have fewer initial pores and also a longer time is required for galvanic penetration to reach the substrate.

Therefore, in commercial operation every effort is made to use at coating as thick as economic limitations will permit. A proper thickness maybe achieved either through the use of nickel alone or nickel used in conjunction with an undercoating metal such as copper. It has been found, however, that heavier deposits of copper under nickel are not always beneficial. Apparently, the cop-per next to steel or a zinc die casting or an aluminum substrate will frequently create a galvanic action, through pores which are present, which is greater than the nickelsubstrate galvanic action.

It would appear that a heavy coating of nickel alone would provide maximum corrosion resistance. However, commercially plated articles, particularly die castings, have intricate contours and it is not infrequent that recessed but significant areas will receive several times less nickel plate than the projecting areas. It becomes a problem then, for some complicated shapes, to deposit enough nickel in the low current density areas to meet specifications or to resist corrosion of the basis metal.

It is, therefore, an object of this invention to provide a nickel plating process employing an undercoating which will produce improved thickness distribution with respect to high and low current density areas and correspondingly improved corrosion protection.

It is a further object of this invention to prepare a composite metallic article comprising a basis member coated with an even thickness of a corrosion resistant undercoating and having a nickel coating superimposed thereon.

It is another object of this invention to prepare a composite metallic article comprising a basis member coated with an even thickness of a corrosion resistant undercoating having a nickel coating superimposed on the undercoating and a chromium coating superimposed on the nickel coating.

It has been found that a copper-nickel alloy will provide.

an excellent undercoat for a corrosion resistant nickel plating process. The solution from which this alloy is deposited has excellent throwing power compared to nickel. The throwing power of this solution using the Haring and Blum cell and a distance ratio of 5 varies between 45% and 50%; that of a nickel solution is about zero. A cyanide copper solution usually has a better throwing power than the 45-50% obtained with the alloy, but as explained above the copper undercoating is frequently without value from the corrosion protection standpoint. Therefore, for recessed parts a heavier thickness of alloy can be obtained in the recessed and significant areas than can be obtained with nickel. The adhesion of the alloy to steel or zinc or aluminum is by no means perfect. Therefore, it is desirable to first coat the article with a flash of copper from a cyanide solution or a flash of nickel. If there were no throwing power problem, i.e. no recesses, there would be no advantage in using the high throwing power Cu-Ni alloy. All Ni would be as good.

In general, the process is carried out by electrodepositng on a basis metal selected from the group consisting of iron, zinc, and aluminum, and alloys thereof, a metal undercoat selected from the class consisting of copper and nickel, electrodepositing a copper-nickel alloy on said metal undercoat, electrodepositing a nickel coating on sa d copper-nickel alloy and electrodepositing a chromium layer on said nickel coating, said copper-nickel alloy being electrodeposited from an aqueous plating solution comprising copper sulfate, nickel sulfate and sodium citrate, having a copper-nickel alloy anode disposed therein. While any type of electrodeposited nickel may be employed to coat the copper-nickel alloy surface, the preferred type of nickel is bright nickel. The term bright nickel as used herein is deemed to include such nickel deposits as will yield a high luster surface without the need of special buffing operations. The preferred thicknesses of the various electrodeposited coatings are :as follows:

The copper-nickel alloy may be deposited by any of these methods which will exhibit better throwing power than can be obtained from nickel solutions such as, for instance, the method given in U .8. Patent No. 1,750,092. The preferred method, however, is that outlined by B. H. Priscott (Transactions of the Institute of Metal Finishing 36, 93-96 (1958-9)).

An example of a specific copper-nickel alloy bath which may be employed is as follows:

Grams per liter 4 It should be understood that copper-nickel alloy anodes, other than 30% nickel anodes, will yield improved corrosion resistance and are contemplated by the invention.

Sodium chloride 2 Any copper-nickel alloy anode having from 30% to 75% Sodium citrate 75 5 nickel is suitable. The copper-nickel alloy undercoat de- NiS -6H O 35 POSi C may be from 30% to 75% nickel; 50% being pre- CuSO -H O 15 ferred. H Water-t k 1000 Replicate panels exposed in a semi-industrial atmos- PH, approx phere for nine months showed the same trend as obtained Operating temperature, 150 F. 10 in Corrodkote test. Since the Corrodkote test does not always give the same results as atmospheric tests, this A Wett'mg g Such as the dlheXylFstef 0f Sodium comparison shows that for these systems the Corrodkote sulfosuccinic acid may also be added to increase the umtest is ti f t formity of the deposit. In another test 6" x 1 /2" steel panels were bent prior The above bath was used with a nlckel'copper alloy to plating, so that a recessed area existed. These were (Ni 30%) anode and deposits, which are denoted in plated similarly to those above, using a nickel strike in- Table I, were produced according to the following curstead of a copper strike but omitting No. 3. The averrent densities and bath agitation: age thickness was the same as above but, of course, the

Table l Agitation Ampcras per Still Slow Medium Fast 10 Uniform semi-bright, Ni- Uniform light brick color, Uniform semi-bright Cu- Uniform Cu-eolor matte,

c or. matte. color. Uniform semi-bright, Ni- Uniform bright color, Uniform dark brick (gray) color. Traces of gas matte. color, matte. tpitting near edges and Suzibright, Ni-color. Irregular dark brick, gray,

Gas streaks (matte) inand Ni-eolor, matte. creasing in amount toward top of panel.

A determination of the resistance of sample panels to corrosion was made by the Corrodkote test, and by outdoor exposure. The Corrodkote test is carried out by preparing a special paste, brushing the paste on each of the test panels and maintaining the paste coated panels in a humidity test cabinet which is so operated as to provide :a non-condensing atmosphere. The special paste is prepared by: measuring out 7.0 ml. of a cupric nitrate solution (2.50 grams of cupric nitrate in 500 ml. of water) and placing it in a 250 ml. glass beaker, measuring out 33 ml. of ferric chloride solution (2.50 grams of ferric chloride in 500 ml. of water) and placing it in the beaker with the 7 ml. of cupric nitrate, measuring out 10.0 ml. of ammonium chloride solution (50.0 grams of ammonium chloride in 500 ml. of water) and placing it in the beaker with the 7 ml. of cupric nitrate and 33 ml. of ferric chloride, weighing out 30 grams of kaolin and adding it to the reagents in the beaker and mixing all the ingredients until a homogeneous slurry or paste is obtained.

Four sample panels were prepared as follows for purposes of corrosion determination:

(1) Copper strike-+0.5 mil bright copper+O.5 mil bright Ni-i-Cr (2) Copper strike+0.5 mil copper-nickel (30% Ni) o 0.5 mil bright Ni+Cr (3) Copper strike+l.2 mil copper-nickel (30% Ni) +Cr (4) Copper strike+1.0 mil bright Ni+Cr After a 20 hour corrosion test by the Corrodkote method, the deposits showed the following defects:

Sample No. l-Excessive rust stain and pin-point holes to the steel.

Sample No. 2Perfect panel, no rust whatsoever.

Sample No. 3Good panel despite the fact that the edges were buffed thinner than 1 mil and the panel showed numerous pits along both edges after "buffing. These pits, after Corrodkote, appeared to contain very little rust. Some edge rust corresponded to areas buffed most.

Sample No. 4Moderate amount of rust stain and pinpoint holes.

deposit thickness on the projecting areas were higher and on the recessed areas lower than the uniform thicknesses given above. These Were Corrodkote tested. There was practically no corrosion on the projecting areas of any of the panels, where the deposit thickness was heavy. The results in the recessed areas were as follows:

(1) Excessive rust; (2) Slight rust; (3) Excessive rust; worse than No. 1.

It will be understood that the aluminum would be given any of the preliminary treatments necessary to effect good adhesion of the copper or nickel to the aluminum. The best known of these are the zincate treatment which first applies a very thin coating of zinc on the surface, and the anodizing method which first applies a porous oxide coating on the aluminum surface.

Having thus disclosed my invention, what I claim is:

1. A process for the electrolytic preparation of a cor rosion resistant article comprising electrodepositing on a basis metal selected from the class consisting of aluminum, iron and zinc and alloys thereof, a metal undercoat selected from the class consisting of copper and nickel, electrodepositing a copper-nickel alloy on said metal undercoat and electrodepositing a nickel coating on said copper-nickel alloy, said electrodeposited materials being free of heat treating.

2. A process for the electrolytic preparation of a corrosion resistant article comprising electrodepositing on a basis metal selected from the class consisting of aluminum, iron, and zinc and alloys thereof, a metal undercoat selected from the class consisting of copper and nickel, electrodepositing a copper-nickel alloy on said metal undercoat, electrodepositing a nickel coating on said coppernickel alloy and electrodepositing a chromium layer on said nickel coating, said electrodeposited materials being free of heat treating.

3. A process for the electrolytic preparation of a corrosion resistant article comprising electrodepositing on a basis metal selected from the class consisting of aluminum, iron, and zinc, and alloys thereof, a metal undercoat 0.05 to 0.3 mil thick selected from the class consisting of copper and nickel, electrodepositing a coppernickel alloy 0.3 to 1.0 mil thick on said metal undercoat, electrodepositing a bright nickel coating 0.2 to 1.0 mil thick on said copper-nickel alloy and electrodepositing a chromium layer 0.005 to 0.1 mil thick on said nickel coating, said copper-nickel alloy being free from heat treating and being electrodeposited from an aqueous plating solution comprising copper sulfate, nickel sulfate and sodium citrate having a copper-nickel alloy containing approximately 30% nickel anode disposed therein, whereby im proved thickness distribution of said copper-nickel alloy is obtained.

References Cited by the Examiner UNITED STATES PATENTS 1,077,977 11/1913 Fuller 204-44 1,750,092 3/ 1930 Crawford et al 204-44 1,787,477 1/1931 Hanley et a1 204-41 X 1,949,623 3/1934 Quarnstrom 29l96.3 2,128,550 8/1938 Ford 204-43 2,428,033 9/1947 Nachtman 29-1963 X 10 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

2. A PROCESS FOR THE ELECTROLYTIC PREPARATION OF A CORROSION RESISTANT ARTICLE COMPRISING ELECTRODEPOSITING ON A BASIS METAL SELECTED FROM THE CLASS CONSISTING OF ALUMINUM, IRON, AND ZINC AND ALLOYS THEREOF, A METAL UNDERCOAT SELECTED FROM THE CLASS CONSISTING OF COPPER AND NICKEL, ELECTRODEPOSITING A COPPER-NICKEL ALLOY ON SAID METAL UNDERCOAT, ELECTRODEPOSITING A NICKEL COATING ON SAID COPPERNICKEL ALLOY AND ELECTRODEPOSITING A CHROMIUM LAYER ON SAID NICKEL COATING, SAID ELECTRODEPOSITED MATERIALS BEING FREE OF HEAT TREATING. 